Sets holder to change when bind_to changes (and does not make bind_to change when holder changes).
For the operation to succeed, the GType of holder and bind_to must be the same, with the exception that
any of them can have a GDA_TYPE_NULL type (in this situation, the GType of the two #GdaHolder objects
involved is set to match the other when any of them sets its type to something different than GDA_TYPE_NULL).
If bind_to is null, then holder will not be bound anymore.
Sets holder to change when bind_to changes (and does not make bind_to change when holder changes). For the operation to succeed, the GType of holder and bind_to must be the same, with the exception that any of them can have a GDA_TYPE_NULL type (in this situation, the GType of the two #GdaHolder objects involved is set to match the other when any of them sets its type to something different than GDA_TYPE_NULL).
If bind_to is null, then holder will not be bound anymore.